cd to / before running soong_build .

This lets one avoid any decisions as to when to chdir there during its
execution and leads to better sandboxing because the pwd doesn't leak to
init() functions anymore.

Test: Manual.
Change-Id: I1560da8ed3a621249426f9e8908aa890c21e13ba
diff --git a/apex/apex_test.go b/apex/apex_test.go
index af5fe06..4bcbf72 100644
--- a/apex/apex_test.go
+++ b/apex/apex_test.go
@@ -2513,7 +2513,8 @@
 	var builder strings.Builder
 	data.Custom(&builder, name, prefix, "", data)
 	androidMk := builder.String()
-	ensureContains(t, androidMk, `LOCAL_MODULE_PATH := /tmp/target/product/test_device/vendor/apex`)
+	installPath := path.Join(buildDir, "../target/product/test_device/vendor/apex")
+	ensureContains(t, androidMk, "LOCAL_MODULE_PATH := "+installPath)
 
 	apexManifestRule := ctx.ModuleForTests("myapex", "android_common_myapex_image").Rule("apexManifestRule")
 	requireNativeLibs := names(apexManifestRule.Args["requireNativeLibs"])